I want to preview a major revision of my Dwebsite before I update the IPFS CID to which the mainnet ENS name resolves; this requires locally resolvable names so hyperlinks and resource references work correctly. There are docs available for deploying ENS on a private chain, and also buying ENS names on public testnets, but I have't found any browser config info. I've searched and looked around the IPFS Companion source and settings without success; how do I configure a browser to resolve ENS names outside of the mainnet?
For quick&dirty smoke tests you could run your own version of https://eth.link (sources: coredns-ens for DNS interop+go-ipfs for HTTP gateway part) under some other domain name (eg.
ethtest.tld) and configure it to resolve ENS names using testnet.
Having that in place you can load website using testnet by replacing
eth.link with the suffix of your DNSLink gateway, for example: https://almonit.eth.link →
For testing you can also choose to run coredns-ens without gateway, and leverage DNSLink resolution on public IPFS gateways instead (for example: http://dweb.link/ipns/almonit.eth/), but then you need to ignore TLS errors caused by domain name mismatch.
Ideally ENS community would provide
eth-testnet.link, but until that is a thing, you can run your own.
While discussing the solution proposed by lidel, I found that I could avoid dealing with ENS entirely if I simply switched to using relative URIs everywhere. I think this is the shortest path to what I need (namely a fully functional site preview before I update ENS), but I've marked lidel's answer as correct so their work in answering the question is credited.